Forest Regeneration in the National Capital Region's Parks, 2023: Slow Gains

Forests cover about three-quarters of the land in the national parks of the National Capital Region around Washington, D.C. These Eastern Deciduous Forest tracts are especially valuable for their size and biodiversity. To last, they need regeneration: seedlings and saplings growing up to fill the gaps left when big trees die. Since 2006, the NPS National Capital Region Inventory & Monitoring Network has tracked whether that's happening. In 2023, the answer was: slowly.

The same Rock Creek Park plot in 2018 (top) and 2023 (bottom), as box elder seedlings grew into saplings. Photos by the National Park Service

What's holding forests back

• Too many white-tailed deer, eating young trees.
• Invasive plants.
• Pests, pathogens and climate change.

Together they have reshaped forest structure and, in places, turned forest into thickets of invasive shrubs. Regeneration in most eastern national parks is too low to sustain the forests in the long run.

The stocking index: little change

The stocking index scores each plot on the number and size of its seedlings and saplings — bigger ones count more, since they're likelier to survive. A park has enough regeneration when 67% of its plots are adequately stocked. Since 2006, no park in the region has topped 30%.

• Compared with 2022, small gains came at Monocacy National Battlefield, the western C&O Canal, Manassas National Battlefield Park and Rock Creek Park.
• The best-stocked were the Baltimore-Washington Parkway, the George Washington Memorial Parkway below the Gorge, and Harpers Ferry; the far western C&O Canal, National Capital Parks–East and Piscataway had no adequately stocked plots.

Stocking index, 2019–2023; the dashed line marks the 67% target, and asterisks mark parks managing deer. Image from the National Park Service

Seedlings: rising in most parks

Seedling density rose in most parks, continuing earlier trends — including Antietam, Catoctin, Harpers Ferry, Manassas, Monocacy, Piscataway, Prince William and Wolf Trap. Greenbelt and Wolf Trap had the most seedlings. Most parks that manage deer saw seedling gains — though the stocking index barely moved.

Seedlings per hectare by park, 2009–2023; dashed blue lines mark when each park began managing deer. Image from the National Park Service

Reading the numbers carefully

• More seedlings don't guarantee future trees — it takes years to reach maturity, with threats all along.
• At Rock Creek, the index rose while seedlings fell: at one plot, many box elders grew from seedlings into saplings.
• At Monocacy and the western C&O Canal, gains came from large pawpaws at one plot each — a non-canopy species that can't fill canopy gaps.
• Many new seedlings at Manassas are ashes, likely doomed by the emerald ash borer.
• Neither measure distinguishes native canopy trees from non-native or non-canopy species; a 2023 study of 39 northeastern parks used methods that do.

Parks taking action

• Catoctin Mountain Park, which has removed invasive plants and reduced deer for over a decade, plans to plant more than 5,000 native trees in an area cleared of Japanese barberry.
• George Washington Memorial Parkway is writing an urban forest management plan with Virginia Tech.
• Rock Creek Park and the Rock Creek Conservancy designed a Forest Resilience Framework in 2023.

Low regeneration remains a concern; restoring these forests will take sustained effort.

Sources

Based on "Forest Regeneration 2023," by Hannah Harkness, NCRN Inventory & Monitoring science communications intern, National Park Service; rewritten in hubnx's own words. The source describes its seedling figure as both eleven and eighteen park graphs, so the number of parks shown is not given here.